1. Field of the Invention
The present invention relates generally to a gas flow rate control system used for an internal combustion engine, and more particularly to the gas flow rate control system for controlling recirculation of gas discharged from the engine back to the same engine in accordance with engine operating conditions.
2. Description of the Prior Art
Most automotive vehicles are equipped with a blow-by gas recirculation system by which so-called blow-by gas leaking from engine combustion chamber to a crankcase is recirculated back to the combustion chamber in order to prevent air pollution and engine performance deterioration. This is called a positive crankcase ventilation (PCV) system and includes a PCV valve adapted to control the flow rate of blow-by gas recirculated back to the engine combustion chamber thereby to prevent fuel concentration of air-fuel mixture to be supplied to the engine. Thus, the PCV valve forms a part of a gas flow control system for blow-by gas.
Recently it has been eagerly desired to improve the function of the PCV valve from view points of strengthening emission control, facilitating fuel consumption control and improving idling operation stability. In order to achieve this goal, it is required that the gas flow rate of the PCV valve correspond to the production amount of blow-by gas to be discharged to a crankcase. That is to say, in case the gas flow rate of the PCV valve is too small relative to the production amount of blow-by gas, the whole amount of blow-by gas cannot be sucked through the PCV valve into the engine combustion chamber and accordingly a part of the blow-by gas is emitted into the atmosphere or enters an air cleaner and a carburetor thereby contaminating them. On the contrary, in case the gas flow rate of the PCV valve is too large relative to the production amount of blow-by gas, a relatively large amount of fresh air is taken into the crankcase through a filler cap or the air cleaner. This unavoidably lowers the rate of fresh intake air inducted through an intake air passage relative to the amount of blow-by gas flowing through the PCV valve particularly under low load engine operating conditions.
In this regard, the conventional PCV valve is so configurated to control the flow rate of the gas flowing through the gas flow passage only in response to engine intake vacuum, and therefore cannot control the gas flow rate in accordance with the production amount of blow-by gas, i.e., in accordance with engine operating conditions, thus making impossible precise control of blow-by gas to be recirculated back to engine combustion chamber.